Flooding Method to Battle Foundation Settlement in Collapsible Soil

Dear reader throughout last few posts we were discussing about collapsible soil. Collapsible soil is a problematic soil and foundation settlement is common and sudden in this type of soil. In south western portion of United States, we have faced many problems with this type of soil.

There have several methods to fight with collapsible soil. Of these, we are discussing here about flooding techniques. In this technique building footprint is flooded, alternatively wells can be provided to force water to penetrate through stratum of collapsible soil.

In some collapsible soil there have some amount of clay content or chemical binder that provide its strength initially when the soil get wet the binding property is diminished and soil get collapsed. In case of cohesionless collapsible soil, also water is the key agent that produce collapse in foundation soil.

In flooding techniques when wetting front passes through ground, this soil get densified until it reach equilibrium state. There have some precautions in using this technique like

-If there have adjacent structures that also supported on collapsible soil the flooding or water forcing techniques are not allowed as this may lead to damage to these structures.

-Before ensuring to be the site safe against collapse, subsurface exploration and extensive laboratory testing have to be performed to find out efficiency of this process.

Long-Term Foundation Settlement in Cohesionless Soil

Cohesionless soils are a nonplastic soil like gravels and sands. Some silts are also nonplastic when there have little or no clay content. We know, cohesionless soil especially gravel is free draining soil; so long-term settlement is rare in this type of soil.

In this case settlement of these granular soils primarily is due to compression of soil skeleton under foundation loads. Their compressive load results a rearrangement in soil particles to reach a dense arrangements.
 

So it is clear to us that very loose deposit of sand/gravel when subjected to foundation load suffers more densification due to rearrangement of particles and subsequent settlement than that of dense/very dense cohesionless soil of same type.

It is obvious to us that cohesionless soil usually not suffers long-term settlement. The settlement occurs immediate after/during construction and dense soil results less settlement. Now we have to find out why they are subjected to long-term settlement?

The exceptions where long-term settlement is happened are:

a. Collapsible soil (cohesionless)

b. Seismic loading

c. Vibrations

d. Fluctuation of loads

Collapsible soil

Our concern is collapsible soils that are cohesionless. We have learnt earlier in this blog that collapsible soil suffer large settlement when water infiltrates through the soil under no or small compressive load.

In most cases soil deposit are stratified and different layer, as usual, may have different degree of settlement, here we use the term collapse potential as discussed in our previous post-about collapse potential of collapsible soil.

Total settlement of multiple collapsible layers of soil is derived by adding collapse value of different layers.

Seismic loading

Seismic shaking due to seismic waves traveling through soil layers or along ground surface, may also result densification of cohesionless soil, provided that the soil layer must be in saturated condition and be in loose conditions. This phenomenon is known as liquefaction we have discussed many posts about cause, potential, triggering and mitigation of liquefaction susceptible soil in previous post.

Vibrations

Another source of such settlement is vibration from machinery. Let us consider a machine like printing press supported on foundation soil of this type, here also loose sand is more susceptible. The densification of loose deposit is done slowly by vibration and this is also a long-term settlement.

Fluctuation of loads

These types of cases occur when loading and unloading of material results fluctuation of loads on cohesionless soil. Dear reader in our last part we have discussed this elaborately, please read this post for more clarification.

How do Fluctuating Loads Result Long-Term Foundation Settlement in Cohesionless Soil?

Constraints in Concrete Repair Work

Dear reader in last post we have discussed about structural safety issues in concrete repair work; we will learn here about constraints a repair project. The structural safety is itself a constraint; but how?

The answer is-considering structural safety we have to consider suitable supports to avoid failure of sick concrete structure, which impose an accessibility problem and also interrupt operation of existing facilities.

With structural safety some limitations may imposed by owner of respective structure, even from neighbor building in case of their safety issues and by contamination or disturbance by sound or dust. So an engineer has to be very careful in every steps of this repair operation.

The constraints as follows

• Access to structures to be repaired


• Conflict with operation schedule of structure for both commercial/industrial structures; in case of residential structures often have to be depleted


• Owners limitation


• Weather


• Design life of repaired structure


• Noise during concrete removal and repair


• Hazardous waste

Sometimes existing materials in structure and also repair material may produce constraint in concrete repair work. Environmental considerations are sometimes monumental or minimal for a particular project.

Many of these issues discussed above like noise, waste, design life etc. may be monitored by local agency according to law and owner himself also. Dear reader in our next post we will learn about basis of selection of repair method.

Structural Safety of Concrete Repair

Dear reader, concrete repair is specialized job and should be performed carefully by professional repair person with supervision and monitoring of knowledgeable engineer. Safety issue is very important in during concrete repair and also before or after repair. We will learn about structural safety in this post.

Considering cost and risk of concrete repair we should always try to avoid repair with possible adjustment in
 

• Concrete exposure

• Drainage

• Foundation soil

• Reducing service load

Dear reader we have already discussed about service load reduction in respect of concrete repair.

Deterioration of concrete member may involved in concrete deterioration itself and reinforcement embedded in it and with concrete, in many times, reinforcing steel have to be removed from concrete member.

Thus when section is reduced, the shear strength along with bending capacity and in applicable cases tensile strength or compressive strength is also reduced. With this section reduction when reinforcement is also removed, the situation often becomes very critical.

We know, in concrete members, cast monolithic, have always redistribution of loads/stresses; the repair engineer should have structural insight to understand this and possible support (temporary) like shoring, bracing and in applicable cases strengthening should be considered.

Structural review, in applicable cases, should include

• Volume change with changes in temperature

• Dead loads and live loads

• Point of special attention like

• Sections located at negative moment in beams and slab
• Cantilever beams
• Column
• Spandrel beams
• Connection and joint details

Evaluation of Present Condition of Concrete to be Repaired

Dear reader we have discussed many posts about repair of concrete; here we will learn about evaluation methods to have idea about present condition of concrete that have some sort of deficiencies. Repair or restoration process only under taken when effective repair method and also economical solution is available to make concrete structure or member serviceable or elongate the service life of concrete.

The first step of repair of concrete structure work is to determine present condition of it. Here we will include some review and tests to be conducted for evaluation purposes. We can use more than one and in most case several tasks as followings to have proper evaluation:

• Review of design document available for concrete structure, which is not often available in case of old construction.


• Review of construction document of the same



• Review on structural-instrumentation data


• Review of documentation or any type of record that provide information about repair work conducted before this session


• Review of any type of maintenance work; if there have any records


• Visual examination


• Destruction testing like core drilling


• Nondestructive testing like rebound hammer test, as discussed in previous posts.


• Laboratory test of concrete samples

Dear reader the steps stated above should be conducted each if possible, but when sufficient records and documentation are not available, as much as steps should be conducted to have proper evaluation.

After completion of all steps, an engineer has through understanding about present condition of concrete and also has insight about causes of distress and deterioration of concrete.

Which to Deal Cause or Symptom for Concrete Repair?

A basic realization of cause underlying deficiencies in concrete is essential for proper evaluation of meaningful repair. If we can determine or understand cause of particular deficiencies, it is easy to choose appropriate repair techniques which leads successful restoration of concrete structure or member. Obviously proper repair will provide maximum life of repair which is the main objective of this restoration process.

Now come to the point-‘symptoms’ and ‘cause’ of repair. Symptoms are deficiencies of concrete that is can be observed under naked eyes in most case they appeared as cracking, spalling, corrosion, discolor of concrete surface and access of fluids etc.

In most case, they are consequence of internal/external causes. So symptoms/observations of deficiencies should be properly differentiated from actual cause or causes of deficiencies.

Sometimes more than one cause may result one symptom and if we treated for one causes and another causes left untreated, the result will be temporary repair and very soon untreated cause will be expressed as some symptom.

Sometimes one cause of deficiency results another deficiency say a case that cause crack may lead to corrosion of reinforcement, which will lead structural cracking and also dealmination of concrete. Dear reader we have provided information about cracking, spalling and delamination of concrete in our previous posts.

So it is advisable to deal with causes not with symptoms as most case it lead to failure of repair process. Only when we know the cause/causes, we can make decisions about rationale repair system.

Example: we have to select repair method for cracking-

Cracking may occur due to:

 Accidental overloading

 Drying shrinkage

 Freezing and thawing cycles in one word alternate thermal cycles

 Improper construction

 Inadequate design

 and may from many reasons

The symptom, cracks can be resulted from above reasons if you treated for drying shrinkage and cause lies in freezing and thawing or due to expansion of foundation soil/shrinkage of the same(not listed above), the repair work will be failed.

Factors Controlling Performance of Concrete against Fire

Dear reader we have learnt that concrete is safer than other construction material in respect of fire. Here our concern is that to learn factors which influence concrete performance.

Many factors can be summarized which control response of concrete against fine as follows:

a. Composition of concrete

b. Permeability of concrete

c. Size of member

d. Temperature gradient (rate of increase)

e. Other factors

Concrete consists of cement paste and aggregates. Both these components of concrete contain components which may be decomposed in contact of high temperature. Dear reader under which temp which hydration product will decompose with be discussed in next post.

Aggregate containing quartz and carbonate decomposed or transformed and should carefully be studied; we will learn these in our upcoming posts.

While decomposition is occurred, internal pressure are developed due to formation of different gases and in this regard the important factor is 

-Permeability

-Member thickness

-Rate of heat transferred to concrete

The loading on concrete member is also important. The test conditions under which specimen are exposed to fire influence the development of microcracking of concrete i.e. whether the concrete is loaded or not when exposing to fire.

Actual performance of concrete subjected to high temperature is influenced by many simultaneously acting factors which is very difficult to interrelated to have precise analysis, we will try learn these factors in microstructural point of view.

Why does Concrete Building Inherently Offer Fire Safety?

Dear reader in our previous post we have discussed about superiority of concrete over other construction material; here we also learn this term but in context of fire safety. When residential, commercial or industrial buildings are designed, human safety is one of the most important considerations and fire is a life threatening hazard. In Skyscrapers construction, evacuation during fire is critical consideration as a part of building safety.

In this regard concrete has records to perform well. Construction material like wood and plastic are combustible and in sometimes generate toxic fume when exposed to high temperature. Dear reader it should keep in mind that many people are died during fire not by burning but due to lack of oxygen or due to smoke.

Unlike above construction material concrete doesn’t release toxic fume and also incombustible. When compared to steel, even at 700~800⁰C temperature concrete shows adequate strength for considerable long periods. The time is important as evacuation or rescue operation can be done within this time.

Let us consider an example; in 1972 in Sao Paulo a 31 storied building (RCC frame) while maintenance work for structural integrity was exposed to fire of high intensity for more than 4 hours and the rescue operation had successfully finished within this time (saved over 500 persons).

Dear read in upcoming posts we will discuss fire performance of normal and high performance concrete and their microstructural behavior with high temperature.

Relation between Method of Compaction and Dry Density of Soil

Dear reader in previous three posts we have discussed about influence of water content and type of soil on dry density. We like to include here that methods of compaction have also great influence on dry density.

With same compaction effort, different compaction method result different dry density of foundation soil. When compaction effort is constant, the dry density depends upon following actions utilized by compaction process.

a. Kneading action

b. Dynamic action

c. Static action

Let us take an example of Harvard Miniature compaction test. We have chosen this test as there as compaction effort kneading action is applied.

In this test, 0.5” diameter tamping foot of cylindrical shape is used. The mould has capacity 1/450 ft3. The force required for tamping, Nos. of layers required for tamping for each layer depend on type of soil and required degree of compaction.

In this test, compaction curve developed have different shape from the curve found in conventional tests, but compaction effort is equal in developing both curve.

As conventional test we mean standard proctor test, modified proctor test etc.

So compaction curve is different for different method of test and consequently lines connecting optimum will be also different. Dear reader in our next post we will learn about lines of optimum and zero void lines in compaction curve of soil.

How do Soil Types Influence Compactive Effort?

The purpose of compactive effort is to produce a soil mass of maximum dry density. Dear reader in our last post we have discussed about water content and dry density relation. Generally coarse-grained soils under compaction can be reached higher dry density than fine grained soil.

In fig below maximum dry density of well graded sand, low plastic silt, low plasticity clay and high plasticity clay are shown.

When a small amount of fine particles is added to coarse-grained soil, it gain a much better dry density for equal compactive effort. The voids of coarse-grains are filled with the fines resulting less void and high dry density.

When fines are reached such amount that additional fines are available after filling all voids of coarse grained soil, obviously the dry density decreases. So well graded sand obtains higher dry density than gap graded or uniform graded sand; (in one word poorly graded sand).

There have higher air void in cohesive soil relative to cohesionless soil. So cohesive soils have smaller maximum dry density than that of cohesionless soils. More water is required in cohesive soil as compared to cohesionless soils. So optimum moisture content (OMC) is also higher for this soil.

Heavy clays having much higher plasticity definitely have very little dry density and also have much higher optimum moisture content.

Electric Double Layer Theory and Dry Density of Soil

In our last post we have learnt how dry density of soil is related to water content in compaction process. Here we will discuss this phenomenon in light of electric double layer theory and soil structure. Lambe in 1958 provided this relation as discussed below.

At first we will discuss about adsorbed water as it is related to this discussion. We know in some soil surface, there have electro-chemical charges which can hold water. This electro-chemically bond water is adsorbed water. As electrical force act on this water, the behavior of adsorbed water is quite different from normal water.

Plasticity of cohesive soil is due to this adsorbed water. Dear reader in our upcoming post we will learn in details about adsorbed water. Here we want to co-relate its influence on changes in dry density.

In this part we simply explain the phenomena like – the attraction forces in layer of adsorbed water are large when water content is low and produce more resistance against movement of soil particles thus wasting compacting effort.

When water content in soil mass in increased, the repulsive forces remain in particles are increased to allow sliding over one another and finally lead to closely packed condition.

We think you are not satisfied with this explanation, to know how do actually repulsive force developed please read next part, will be published consecutively.

Let consider a cohesive soil deposit to be compacted. In cohesive soil attraction force between adjacent soil particles is defined as van der waals’ force. Dear reader we have already learnt about adsorbed water which produces repulsive forces.

When double layers of this water come close to each other the repulsive force is generated. Repulsive force between two layers has direct relation with size of double layers; but attractive force remains identical at the same magnitude.

If the resultant of these two forces is attractive, a flocculated structure is found. But if resultant is repulsive, our purpose of reducing resistance of movement is achieved i.e. particles will have tendency to move away i.e. dispersed.

When water content is low, double layer producing repulsive force do not developed completely; thus attraction force governs over repulsion. Thus more compaction effort is required to reach a certain degree of compaction and the consequence is low dry density of compacted soil.

With the increase in water content, double layer is expanded to increase repulsive forces. Now the particles can easily slide past one another reducing compacting effort and becomes packed easily and more closely. The consequence is high dry density.

The double layer’s expansion is not infinite. At optimum water content (optimum moisture content, OMC), this expansion process is completed and we can notice from compaction curve that maximum dry density reached at this water content. Beyond OMC, water fills the space between soil grains increasing void in soil mass thus reducing dry density.

Compacted Fill Materials for Foundation Trench (IBC)

We know foundation sometimes rest on compacted fill. In case of questionable soil after sufficient test foundation engineer sometimes suggest to remove this soil and replace the excavated soil with fill material providing sufficient compaction. Dear reader we have used the term problematic soil which is defined in IBC as questionable soil.

In IBC seismic site classification, site class F is defined by this type of soils. The questionable soils are expansive soil, collapsible soil, soluble soil etc. we have published many post about these types of soil.

When a shallow foundation is supported on compacted fill having thickness more than 305 mm (12 inch), the geotechnical investigation is essential and which include the following requirements:

a. Specification to prepare site before placing fill material and subsequent compaction.

b. Specifications for fill material that will be left for compaction.

c. Dear reader we know any compaction process should have accountability and we have two parameters to measure performance

• Optimum moisture content
• Maximum dry density

The test methods for determining these parameters should be defined in investigation.

d. Allowable maximum thickness of the each layer of fill materials

e. Test method to determine in-situ dry density in the field

f. Minimum allowable in-situ dry density which is generally represented as percentage of maximum dry density from item (c) known as MDD.

g. Required frequency and number of in-situ tests to check minimum allowable in-situ dry density.

What are the Advantages of Large Pores in No-fines Concrete?

Omitting fine aggregate in no-fine concrete, a porous concrete is achieved. The modification of concrete is done to have some advantage and to conserve valuable materials. Dear reader we here will discuss about intention of providing large pores in this concrete.

The large pores allow no capillary action within no-fines concrete. This concrete shows high resistance against frost action when they are not saturated; this is due to absence of capillary suction.

But when pores are saturated, freezing will produce rapid disintegration. The pores in this concrete, offer as a great absorption of sound which produce great experience in sound acoustic system.

As discussed in previous posts, no or least compaction is required in placing this concrete; more over this concrete have no tendency to segregate. The pore system within concrete produces a lighter concrete but at the cost of low strength.

The shrinkage of no fines concrete is significantly lower as compared to ordinary concrete. The thermal expansion of this concrete is also lower than ordinary concrete.

The almost free draining property of no fines concrete allows it to use in applications where free drainage is required as an example we can include- permeable concrete to allow water to penetrate subgrade and soil in gardening around pavement. We have published some posts about previous concrete in our previous posts; we can go through this for more information.

Requirements for Geotechnical Investigation for Deep Foundation (IBC)

Dear reader, scope of geotechnical investigation is defined by registered professional involved in foundation design. They also define types of boring/sounding and numbers too, sampling equipment, drilling equipment, test program in laboratory and equipment for in-situ testing, if required.

In case of deep foundation, geotechnical investigation is more important, as cost involvement in drilling and construction of foundation is much higher than shallow foundation and right method of drilling and selection of foundation type are mostly dependent on proper investigation.

Unless adequate data are available depending on which foundation design & installation can be performed, IBC recommended to investigate including following:

a. Recommendation for types of deep foundation and also installed capacities.

b. Recommendation for spacing of foundation element (spacing means distance measured center to center).

c. Driving criteria

d. Procedures of installation

e. Procedures of inspection and reporting of field performance.

f. These are essential to verify installed capacity of foundation element.

g. Requirements of load testing.

h. Suitability of foundation materials considering ambient environment (durability perspective).

i. Designation of the bearing strata

j. Recommendation for defining group capacity of deep foundation elements generally reduction is experienced in group action.

Code also provided for requirements for doubtful characteristics in rock structure. When subsoil profile at project site shows variations in characteristics, to avoid confusion additional boring should made up to a depth equal or greater than 10 ft below foundation level. This provides assurance of soundness of foundation bed & bearing capacity as well.

Application of No-Fines Concrete

No-fines concrete have porous structure and as discussed in previous post they have lower strength than ordinary concrete. In this post we will learn about application of no fines concrete.

They have air permeability and high water absorption. They can be used in pavement where vegetation or trees are surrounded by road. The permeability of no fines concrete allows water to flow through them.

This easy drainage facility of no fines concrete is also used in domestic parking. This type of parking facility is essentially supported on permeable subgrade.

No fines concrete is mainly used in load bearing wall of domestic buildings. In case of framed structure, they can be used in infill walls.

Normally reinforcement is not included in no fines concrete; if required treatment of surface of reinforcement is required.

The treatment is done by providing coating with thin layer of cement paste. The layer recommended is about 1/8 in. or 3 mm thick.

This treatment not only prevents corrosion but also increase bond between reinforcement and surrounded porous concrete. Due to large pores in concrete the contact surface between reinforcement and concrete is very low. This treatment is thus essential for reinforcing no fines concrete.

The coating can be provided easily by shotcreting onto reinforcement. Dear reader we have discussed many information about shotcrete in our previous posts.